Low temperature error thermal sensor

ABSTRACT

A thermal sensor in some embodiments comprises two temperature-sensitive branches, each including a thermal-sensing device, such as one or more bipolar-junction transistors, and a current source for generating a current density in the thermal-sensing device to generate a temperature-dependent signal. The thermal sensor further includes a signal processor configured to multiply the temperature-dependent signal from the branches by respective and different gain factors, and combine the resultant signals to generate an output signal that is substantially proportional to the absolute temperature the thermal sensor is disposed at.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 62/738,292, filed Sep. 28, 2018, and titled “Low Temperature Error Thermal Sensor,” the disclosure of which is hereby incorporated herein by reference.

BACKGROUND

This disclosure relates generally to thermal sensors. This disclosure more specifically relates to reducing temperature errors for thermal sensors.

Thermal sensors find a wide range of uses. Accuracy of thermal sensors is important, or even critical, for many applications. Where a thermal sensor is calibrated at only one or limited number of temperatures, the accuracy of the thermal sensor over the full range of intended use may be difficult to ensure due to deviations of the sensor characteristics from ideal characteristics. Efforts are ongoing in improving thermal sensor accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1A schematically shows component thermal sensors in a thermal sensor according to one aspect of the present disclosure.

FIG. 1B is an illustration of ideal and actual voltage-temperature relationships for each of the component thermal sensors in FIG. 1A according to one aspect of the present disclosure.

FIG. 2A is an illustration of approximating the voltage-temperature relationship in an operating temperature range for a component thermal sensor according to one aspect of the present disclosure.

FIG. 2B illustrates modified relative voltage-temperature relationships for two component thermal sensors matching offsets according to one aspect of the present disclosure.

FIG. 3 schematically illustrates a discrete-timing-type thermal sensor circuit according to one aspect of the present disclosure.

FIG. 4 schematically illustrates a continuous-DC-type thermal sensor circuit according to one aspect of the present disclosure.

FIG. 5 schematically illustrates a continuous-DC-type thermal sensor circuit with differential feedback according to one aspect of the present disclosure.

FIG. 6 schematically illustrates a continuous-DC-type thermal sensor circuit with single-ended feedback according to one aspect of the present disclosure.

FIG. 7 shows temperature performance data at various process corners for prototype thermal sensors of the type shown in FIG. 6.

FIG. 8A illustrates voltage-temperature relationships for two transistors in a differential current-density thermal sensor and fractional slope adjustment of the voltage-temperature relationship of one of the transistors according to one aspect of the present disclosure.

FIG. 8B illustrates differential-voltage-temperature relationship for the thermal sensor having the voltage-temperature relationships shown in FIG. 8A.

FIG. 9 schematically illustrates a discrete-timing-type thermal sensor circuit with fractional slope adjustment according to one aspect of the present disclosure.

FIG. 10 schematically illustrates a continuous-DC-type thermal sensor circuit with resistor-ratio fractional slope adjustment according to one aspect of the present disclosure.

FIG. 11 schematically illustrates a continuous-DC-type thermal sensor circuit with resistor-ratio and op-amp-driver fractional slope adjustment according to one aspect of the present disclosure.

FIG. 12 schematically illustrates a continuous-DC-type thermal sensor circuit with V_(be) replica fractional slope adjustment according to one aspect of the present disclosure.

FIG. 13 schematically illustrates a switched single-transistor-branch thermal sensor circuit with fractional slope adjustment controlled by a digital control unit (e.g., microcontroller (“MCU”)) according to one aspect of the present disclosure.

FIG. 14 shows temperature performance data at a process corner for prototype thermal sensors of the type shown in FIG. 11.

FIG. 15 schematically illustrates a continuous-DC-type thermal sensor circuit with differential feedback, similar to the circuit shown in FIG. 5, but with the bipolar-junction transistors replaced with field-effect transistors, according to one aspect of the present disclosure.

FIG. 16 outlines a method of temperature measurement according to an aspect of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Thermal sensors, or temperature sensors, are widely used in a variety of applications. For example, thermal sensors are used in integrated circuits, such as memory modules, to monitor and control the temperature of the integrated circuits to ensure their proper operation. Examples of thermal sensors include sensors employ bipolar junction transistors (“BJTs”), for which the voltage between terminal pairs (such as base-emitter voltage, or V_(BE)) has certain temperature dependency that has certain characteristics, such as exponential dependency in ideal (theoretical) cases. Deviations from the ideal characteristics cause errors in temperature measurements. Certain embodiments disclosed herein have reduced temperature errors than conventional thermal sensors.

In some embodiments, a temperature sensor includes two pairs of BJTs. In each pair, the two BJTs have different current densities, either by passing the same current through two BJTs of different sizes, or passing different currents through two BJTs of the same size, or a combination of the two methods. Each BJT produces a V_(BE) as the result. The differential V_(BE) (dV_(BE), or ΔV_(BE)), or the difference between two V_(BE)'s, in each pair, is obtained, and the difference between dV_(BE)'s of the two pairs is taken as the output signal. The dV_(BE)'s from the pairs can be amplified differently so that the dV_(BE)'s has substantially the same offset value (S1) in a parameter, such as the offset voltage at a reference temperature (e.g., OK), where the offset voltage can be defined as the offset voltage, at the reference temperature, of the tangent line (or some other linear approximation of the dV_(BE)-temperature curve within the operating temperature range) of the dV_(BE)-temperature curve at a calibration point (e.g., 25° C.). Other forms of approximations of the voltage-temperature relationships can also be used. Other types of transistors, such as MOSFETs, can be used instead of the BJTs.

In some embodiments, a single pair of BJTs can be used, but the V_(BE)'s can be amplified differently such that the amplified V_(BE)-temperature curves match closely over the intended operational temperature range.

Referring to FIG. 1A, two BJT pairs (110,140) are configured to generate the dV_(BE)'s (dV_(BE1) and dV_(BE2), respectively), which are to be combined to provide a voltage signal that is closer to having a more desirable temperature dependency than either of the BJT pairs (110,140). A more desirable temperature dependency can by, for example, a linear dependency in which the voltage is substantially proportional to absolute temperature (“PTAT”), such that a single-point temperature calibration (e.g., at 25° C.) is sufficiently reliable to temperature sensing throughout the intended range of temperatures (e.g., from about −50° C. through about 150° C., −40° C. through about 125° C., from about −25° C. through about 110° C., or from about 0° C. through about 100° C.) for the operation of the electronic device.

Each BJT pair (110, 140) in the example illustrated in FIG. 1A includes two branches (120, 130 in the pair (110), and (150, 160) in the pair (140)) connected to each other in parallel. Each branch includes a current source (122, 132, 152, 162) connected in series with a diode-connected transistor (124, 134, 154, 164), respectively). The base and collector in each of the diode-connected transistors are connected to each other and connected to a common reference point (in this example, ground); the emitter is connected to the currently source.

Each diode-connected transistor (124, 134, 154, 164) is designed to operate at its respective emitter current density. The current density in each diode-connected transistor is determined by the current in the respective branch and the cross-sectional area n, m, p, or q, of the emitter-base junction (“emitter area”) of the diode-connected transistor. Thus, different current densities can be produced by currents of different amplitudes from the current sources (122, 132, 152, 162) with the same cross-sectional areas for the emitter-base junctions in the diode-connected transistors (124, 134, 154, 164). Different current densities can also be produced by currents of the same amplitude from the current sources (122, 132, 152, 162) but different emitter areas n, m, p, and q, respectively, for the diode-connected transistors (124, 134, 154, 164). In a further alternative, different current densities can be produced by the currents of the different amplitudes from the current sources (122, 132, 152, 162) with different cross-sectional areas for the emitter-base junctions in the diode-connected transistors (124, 134, 154, 164).

Given the different current densities in the two branches of each BJT pair (110, 140), a differential voltage is generated between the emitters of the two diode-connected transistors in the BJT pair as a function of the temperature the two diode-connected transistors are at. Thus, a differential voltage dV_(BE1) is generated between the emitters of diode-connected transistors (124, 134); a differential voltage dV_(BE2) is generated between the emitters of diode-connected transistors (154, 164).

Referring to FIG. 1B, for each BJT pair (110, 140), the differential base-emitter voltage (dV_(BE21) or dV_(BE2)) has it temperature dependency. The relationship between the differential base-emitter voltage (dV_(BE) (=dV_(BE21) or dV_(BE2))) and current densities,

${{dV}_{BE} = {{CT} = {\frac{\eta\;{kT}}{q}\ln\; r}}},$ where T is absolute temperature at the BJT pair, η is an ideality factor, k is the Boltzmann constant, q is the electron charge, r is the ratio between the emitter current densities in the BJT pair, and

$C = {\frac{\eta\; k}{q}\ln\;{r.}}$ In cases where the currents in both branches of each BJT pair are equal, r=n/m and r=p/q, respectively.

For ideal thermal sensors, in which η is a constant for all temperatures, C is a constant, V_(BE) is a proportional to the absolute temperature, T, and the plot of dV_(BE) as a function of temperature is a straight line passing through the origin, i.e., dV_(BE)=0 V at T=0 K. Thus, for the BJT pairs (110, 140), the ideal dV_(BE1)-T and dV_(BE2)-T plots would be straight lines (170, 180), respectively, in FIG. 1B. In actual thermal sensors, however, the ideality factor, is typically not constant with temperature. Consequently, the dV_(BE)-T relationship would typically not be linear, as indicated by curves (172, 182) for the BJT pairs (110, 140), respectively.

Referring to FIG. 2A, and using the BJT pair (140) as an example, for an ideal BJT pair, a PTAT relationship for dV_(BE2) would result in a straight line (210) in a dV_(BE2)-T plot, as discussed above. In such a case, a single-point calibration, i.e., calibration by measuring dV_(BE2) at a single temperature, such as 25° C., would result in accurate temperature measurements because a single constant, C (or the slope of the straight line (210)), would be needed to determine all values of dV_(BE2) at all temperatures. However, as C is typically not a constant with temperature, the actual dV_(BE2)-T plot is typically a curve, such as the curve conceptually illustrated (182). As a result, a single-point calibration (e.g., at 25° C.) would result in errors, in some cases significant, at temperatures away from the calibration point (e.g., ε₁ at −40° C. and ε₂ at) 125°.

In practical applications, accuracy of calibration of temperature sensors is of concern only for the intended temperature range of operation, such as from about −40° C. to about 125°. In accordance with certain aspects of the present disclosure, differential base-emitter voltages from two BJT pairs (110, 140) can be appropriately combined to obtain a signal that is substantially PTAT at least in the intended temperature range of operation, such that single-point calibrations are adequate for the intended operation of the electronic device. For example, in FIG. 2A, the dV_(BE2)-T curve (182) may be adequately approximated by a straight line (220) in the temperature range from about −40° C. to about 125°. The straight line (220) may be based on any approximation method that results in errors within an acceptable level over the intended temperature range of operation. For example, the line (220) can be the tangent line to the dV_(BE2)-T curve at the calibration temperature (e.g., 25° C.); in another example, the line (220) is the line connecting the two ends (e.g., at −40° C. and 125°) of segment of the curve (182) over the intended temperature range; in a further example, line (220) can be a least-square fit to the curve over the intended temperature range.

Similarly, as shown in the dV_(BE1)-T curve (172) can be approximated by a straight line (not shown in the drawings) over the same temperature range as for the dV_(BE2)-T curve (182). The line (220) for the dV_(BE2)-T curve (182) in this case has an intersect S₁ on the y-axis (i.e., 0 K); the line for the dV_(BE1)-T curve (172) in this case has an intersect S₂ on the y-axis. According to an aspect of the present disclosure, dV_(BE1) can be amplified by a factor, M (which can be greater, equal, or smaller than 1), such that M·S₂=S₁. As shown in FIG. 2B, the line (280) that approximate the amplified differential voltage curve (272), M×dV_(BE1), has the same intersect, S₁, as the dV_(BE2)-T curve (182). The differential voltage, dV_(BE12), is thus of the form, dV _(BE12) =M·dV _(BE1) −dV _(BE2) ≈C′T+M·S ₂ −C″T−S ₁=(C′−C″)T, where C′ and C″ are constants. The combined differential voltage is thus substantially PTAT, and the thermal sensor producing such a differential voltage is suitable for single-point calibration. That is, a differential voltage value, dV₀, can be obtained ad dV_(BE12) at a single, known temperature, T₀ (e.g., 25° C. (or 298 K)), and the temperature, T, can be determined as T=dV·T₀/dV₀, wherein T and T₀ are measured in K. dV _(BE12) =M·dV _(BE1) −N·dV _(BE2,) is substantially a PTAT signal.

Thus, to obtain a substantially PTAT signal in a temperature measurement, the following steps can be taken, according to some embodiments: generating a first current density in a first thermal-sensing device disposed at a temperature, T (1610); generating a second current density in a second thermal-sensing device disposed at T (1620), the second current density being different from the first current density; generating a third current density in a third thermal-sensing device disposed at T (1630); generating a fourth current density in a fourth thermal-sensing device disposed at T, the fourth current density being different from the third current density (1640); obtaining a first differential voltage between a first voltage generated by the first thermal-sensing device responsive to the first current density and T on the one hand, and a second voltage generated by the second thermal-sensing device responsive to the second current density and Ton the other (1650); obtaining a second differential voltage between a third voltage generated by the third thermal-sensing device responsive to the third current density and T on the one hand, and a fourth voltage generated by the fourth thermal-sensing device responsive to the fourth current density and T on the other (1660); obtaining a third differential voltage, dV, between the first differential voltage multiplied by a first gain factor and the second differential voltage multiplied by a second gain factor, the second gain factor being different from the first gain factor (1670); and determining T based on the third differential voltage (1680).

Referring to FIG. 3, a thermal sensor circuit (300) according to one aspect of the present disclosure implements the scheme described above for obtaining a substantially PTAT thermal sensor. In this example, the circuit (300) is a discrete-timing type circuit. It includes the BJT pairs (110, 140) described above, and a signal processing circuit, which in this example is a pair of switched-capacitor amplifiers (310, 340). The switched-capacitor amplifier (310) includes an op-amp (312), an input capacitor (314), a feedback capacitor (316), and switches (318, 320, 322), which can be any suitable switching devices, including switching transistors. Similarly, the switched-capacitor amplifier (340) includes an op-amp (342), an input capacitor (344), a feedback capacitor (346), and switches (348, 350, 352), which can be any suitable switching devices, including switching transistors. The switched inputs of the amplifier (310) are connected to the respective emitters of the diode-connected transistors (124, 134) to receive the differential signal dV_(BE1), which is the difference between the voltage V_(BE1a) at the emitter of the diode-connected transistor (124) and the voltage V_(BE1b) at the emitter of the diode-connected transistor (134). Similarly, the switched inputs of the amplifier (340) are connected to the respective emitters of the diode-connected transistors (154, 164) to receive the differential signal dV_(BE2), which is the difference between the voltage V_(BE2a) at the emitter of the diode-connected transistor (154) and the voltage V_(BE2b) at the emitter of the diode-connected transistor (164).

The amplifiers (310, 340) in one example have gains M and N, respectively, where M·S₂−NS₁=0, where S₁ and S₂ are, respectively, the y-intersects of the lines approximating the dV_(BE2)-T curve (182) and dV_(BE1)-T curve (172) in the intended temperature range of operation (e.g., from about −40° C. to about 125° C.). The differential output signal, dV_(BE12), is, as discussed above, substantially proportional to the absolute temperature within the intended temperature range of operation. The thermal sensor (300) is thus susceptible to single-point calibration.

As used in the present disclosure, “gain” or “amplification” connotes a factor by which a signal input into a circuit (“amplifier”) is multiplied to generate the output of the circuit; the factor is not limited to numbers greater than 1.

Other discrete-timing type circuits, such as switched-capacitor sample/hold circuits, can also be used.

Signal processing circuits for obtaining different gains for the two BJT pairs can be of any kind suitable for such purpose. For example, a continuous DC type circuit can also be used. In FIG. 4, for example, a continuous DC type amplifier (400) can be used instead of the switched-capacitor amplifiers (310, 340) shown in FIG. 3. The amplifier (400) includes a pair of summing amplifiers (410, 430) and a differential amplifier (450). The first summing amplifier (410) includes an op-amp (410), two input resistors (414, 416) and a feedback resistor (418). Similarly, the second summing amplifier (430) includes an op-amp (430), two input resistors (434, 436) and a feedback resistor (438). The differential amplifier (450) includes a pair or op-amps (452, 454), an input resistor (456) and feedback resistor (458) associated with the op-amp (452), and an input resistor (460) and feedback resistor (462) associated with the op-amp (454). The summing amplifiers (410, 430) receives two pairs of inputs: V_(BE1a) and V_(BE2b) for the amplifier (410), and V_(BE2a) and V_(BE1b) for the amplifier (430). The outputs of the summing amplifiers (410, 430) are inputs to the differential amplifier (450). The output, ΔV, of the differential amplifier (450) is a linear combination of the inputs V_(BE1a), V_(BE2b), V_(BE2a), and V_(BE1b). With appropriate selection of the resistance values of the resistors, ΔV can be set to a linear combination of dV_(BE1) and dV_(BE2). For example, with the resistance values symbolized in FIG. 4, ΔV=M·dV_(BE1)−dV_(BE2) for R₁=R₂=R₃=R₄ with M being a value that makes ΔV substantially proportional to the absolute temperature (e.g., M·S₂−S₁=0).

Referring to FIG. 5, a thermal sensor circuit (500) according to another aspect of the present disclosure includes two branches of BJT pairs (510, 540). The first BJT pair (510) includes a pair of diode-connected transistors (524, 534), and current-bias transistors M1 (522), M2 (526), M3 (532), and M4 (536), where the labels “M1,” “M2,” “M3” and “M4” also denotes the sizes, such as channel widths, of the respective transistors (522, 526, 532, 536). Similarly, the second BJT pair (540) includes a pair of diode-connected transistors (554, 564), and current-bias transistors M5 (552), M6 (556), M7 (562), and M8 (566), where the labels “M5,” “M6,” “M7” and “M8” also denotes the sizes, such as channel widths, of the respective transistors (552, 556, 562, 566). The transistors M1-M8 in this example are field-effect transistors (FET's), such as Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET's) but can be any suitable type transistors. The emitter of the first diode-connected transistor 524) in the first branch (510) is directly connected to the drains of transistors M2 (526) and M5 (552); the emitter of the first diode-connected transistor (554) in the second branch (540) is directly connected to the drains of transistors M6 (556) and M1 (522). The emitter of the second diode-connected transistor (534) in the first branch (510) is connected to the drains of transistors M3 (532) and M8 (566) through resistor R1 (538); the emitter of the second diode-connected transistor (564) in the second branch (540) is connected to the drains of transistors M7 (562) and M4 (536) through a resister R2 (568).

Each branch (510, 540) is associated with a respective amplifier (570, 580). The input of the first amplifier (570) being the differential voltage, dV_(BE1), between the emitters of the diode-connected transistors (524, 534) through resistor R1 (538); the input of the second amplifier (580) being the differential voltage, dV_(BE2), between the emitters of the diode-connected transistors (554, 564) through resistor R2 (568). The output of the first amplifier (570) is connected to the control electrodes (gates in this example) of transistors M1 (522), M2 (526), M3 (532), and M4 (536); the output of the second amplifier (580) is connected to the control electrodes (gates in this example) of transistors M5 (552), M6 (556), M7 (562), and M8 (566).

A first output, V_(out1), of the thermal sensor (500) is provided at the junction between a first output resistor R4 (598) and the drain of a first output transistor M10 (596); a second output, V_(out2), of the thermal sensor (500) is provided at the junction between a second output resistor R3 (594) and the drain of a second output transistor M9 (592). The control electrode (gate in this example) of the first output transistor M10 (596) is connected to the control electrodes of transistors M1 (522), M2 (526), M3 (532), and M4 (536); the control electrode (gate in this example) of the second output transistor M9 (592) is connected to the control electrodes of transistors M5 (552), M6 (556), M7 (562), and M8 (566). The source electrodes of transistors M1 through M10 and connected together. Transistor M10 and resistor R4 form a first current mirror with transistor M1, generating the voltage output V_(out1) from resistor R4 and image current of the current, I2, through the transistor M1. Likewise, transistor M9 and resistor R3 form a second current mirror with transistor M8, generating the voltage output V_(out2) from resistor 3 and image current of the current, I1, through the transistor M8.

In this example, the outputs, V_(out1) and V_(out2), are each a function of dV_(BE1) and dV_(BE2), each of which is a function of ratio current densities in the BJT pair (510, 540). Similar to the configuration shown in FIG. 3, the current density ratio in each BJT pair can be set by the same BJT size (n=m; p=q) but a non-unity ratio (I_(M2):I_(M3)) between the currents through the transistors M2 and M3 (for BJT pair (510)), or M6 and M7 (I_(M6):I_(M7)) (for BJT pair (540)). Alternatively, the current density ratio in each BJT pair can be set by the same currents through the transistors M2 and M3 (I_(M2):I_(M3)=1), or M6 and M7 (I_(M6):I_(M7)=1), but different BJT sizes. A combination of the above two configurations (i.e., different BJT sizes and transistor currents) can also be employed.

The current ratios, I_(M2):I_(M3) and I_(M6):I_(M7), in turn, are functions of dV_(BE1)/R1 and dV_(BE2)/R2, respectively. The current ratios are further functions of ratios of W/L ratios between transistors M1 and M2, M4 and M3, M5 and M6, and M8 and M7, where the W/L ratio for a transistor is the aspect ratio (width:length) of the channel of the transistor. In one example, the ratio of W/L ratios between transistors M1 and M2, and between transistor M4 and M3 is 1:m; the ratio of W/L ratios between transistors M5 and M6, and between transistor M8 and M7 is 1:k. In such a configuration, the output V_(out1) is k₁(m₁dV_(BE1)-dV_(BE2)), where k₁ and m₁ are functions of R1, R2, m, k, and R4; similarly, the output V_(out2) is k₂(m₂dV_(BE2)-dV_(BE1)), where k₂ and m₂ are functions of R1, R2, m, k, and R3. Thus, by appropriate choices of resistance values R1, R2, R3, and R4, and ratios m and k, the voltage offset for dV_(BE2) and/or dV_(BE1) can be set to generate a PTAT voltage, suitable for single-point calibration.

Referring to FIG. 6, a thermal sensor circuit (600) according to another aspect of the present disclosure includes two branches of BJT pairs (610, 640). The first BJT pair (610) includes a pair of diode-connected transistors (624, 634), and current-bias transistors M1 (522), M2 (626) and M3 (632), where the labels “M1,” “M2” and “M3” also denotes the sizes, such as channel widths, of the respective transistors (622, 626, 632). Similarly, the second BJT pair (640) includes a pair of diode-connected transistors (654, 664), and current-bias transistors M4 (656), M5 (662) and M6 (666), where the labels “M4,” “M5” and “M6” also denotes the sizes, such as channel widths, of the respective transistors (656, 662, 666). The transistors M1-M6 in this example are field-effect transistors (FET's), such as Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET's) but can be any suitable type transistors. The emitter of the first diode-connected transistor (624) in the first branch (610) is directly connected to the drains of transistors M2 (626) and M6 (666); the emitter of the first diode-connected transistor (654) in the second branch (640) is directly connected to the drain of transistor M4 (656). The emitter of the second diode-connected transistor (634) in the first branch (610) is connected to the drain of transistor M3 (632) through resistor R1 (638); the emitter of the second diode-connected transistor (664) in the second branch (640) is connected to the drains of transistors M5 (662) and M1 (622) through a resister R2 (668).

Each branch (510, 540) is associated with a respective amplifier (670, 680). The input of the first amplifier (670) being the differential voltage, dV_(BE1), between the emitters of the diode-connected transistors (624, 634) through resistor R1 (638); the input of the second amplifier (680) being the differential voltage, dV_(BE2), between the emitters of the diode-connected transistors (654, 664) through resistor R2 (668). The output of the first amplifier (570) is connected to the control electrodes (gates in this example) of transistors M1 (622), M2 (626), and M3 (632); the output of the second amplifier (680) is connected to the control electrodes (gates in this example) of transistors M4 (656), M5 (662), and M6 (666).

A first output, V_(out1), of the thermal sensor (600) is provided at the junction between a first output resistor R4 (698) and the drain of a first output transistor M7 (696); a second output, V_(out2), of the thermal sensor (600) is provided at the junction between a second output resistor R3 (694) and the drain of a second output transistor M8 (692). The control electrode (gate in this example) of the first output transistor M7 (696) is connected to the control electrodes of transistors M1 (622), M2 (626), and M3 (632); the control electrode (gate in this example) of the second output transistor M8 (692) is connected to the control electrodes of transistors M4 (656), M5 (662), and M6 (666). The source electrodes of transistors M1 through M8 and connected together. Transistor M7 and resistor R4 form a first current mirror with transistor M1, generating the voltage output V_(out1) from resistor R4 and image current of the current, I2, through the transistor M1. Likewise, transistor M8 and resistor R3 form a second current mirror with transistor M6, generating the voltage output V_(out2) from resistor R3 and image current of the current, I1, through the transistor M6.

In this example, the outputs, V_(out1) and V_(out2), are each a function of dV_(BE1) and dV_(BE2), each of which is a function of ratio current densities in the BJT pair (610, 640). Similar to the configurations shown in FIGS. 3 and 5, the current density ratio in each BJT pair can be set by a combination of BJT size (n/m; p/q) and the ratio (I_(M1):I_(M2):I_(M3)) between the currents through the transistors M1, M2 and M3 (for BJT pair (610)), or M4, M5 and M6 (I_(M4):I_(M5):I_(M6)) (for BJT pair (640)). The current ratios, in turn, are functions of dV_(BE1)/R1 and dV_(BE2)/R2, respectively. The current ratios are further functions of ratios of W/L ratios between transistors M1, M2 and M3, and between M4, M5 and M6. Similar to the configuration shown in FIG. 5, in the configuration in FIG. 6, the output V_(out1) is k₁(m₁dV_(BE2)-dV_(BE2)), where k₁ and m₁ are functions of R1, R2, and R4, and the W/L ratios of Ml, M2 and M3; similarly, the output V_(out2) is k₂(m₂dV_(BE2)-dV_(BE1)), where k₂ and m₂ are functions of R1, R2 and R3, and the W/L ratios of M4, M5 and M6. Thus, by appropriate choices of resistance values R1, R2, R3, and R4, and W/L ratios, the voltage offset for dV_(BE2) and/or dV_(BE1) can be set to generate a PTAT voltage, suitable for single-point calibration. As an example, plots of temperature errors as a function of temperature for several process corners (TT, FF and SS) for the thermal sensor shown in FIG. 6 are illustrated in FIG. 7. The single-point calibration is done at 25° C. The temperature errors in the temperature range of −25° C. to 110° C. is within about 3° C. That is, the temperature determined by a straight (PTAT) line passing through (T=0 K, dV_(BE12)=0 V) and (T=273 K+25 K, dV_(BE12) at 25° C.) in a dV_(BE12)-T plot differs from the actual temperature by no more than 3° C. More generally, the output of a thermal sensor is substantially PTAT if the temperature determined by a straight (PTAT) line passing through (T=0 K, dV_(BE12)=0 V) and a calibration point (e.g., T=273 K+25 K, dV_(BE12) at 25° C.) in a dV_(BE12)-T plot differs from the actual temperature by no more than an amount (e.g., 3, 2 or 1° C.) deemed acceptable for the proper operation of the device (e.g., integrated circuit) over an intended range of temperatures.

In another aspect of the present disclosure, instead of combining output signals of two transistor pairs to achieve a more linear temperature dependence, the temperature-dependent voltages from two single transistors can be appropriately combined to obtain a more linear signal, with temperature, than with a single transistor. For example, in one aspect of the present disclosure, the base-emitter voltages (V_(BE)'s) in a single BJT pair, such as the BJT pair (110), can be appropriately combined to produce a signal that is more linear with absolute temperature than the V_(BE) of either transistor (124, 134).

As shown in FIG. 8A, each transistor in a BJT pair, such as the BJT pair (110) shown in FIG. 1A, generates a temperature-dependent V_(BE). Thus, for example, the voltage, V_(BE1), at the emitter of the diode-connected transistor (124) has a V_(BE)-temperature curve (810), and the voltage, V_(BE2), at the emitter of the diode-connected transistor (134) has a V_(BE)-temperature curve (820). The differential voltage, dV_(BE) (840), as illustrated in FIG. 8B, may deviate significantly from the ideal PTAT temperature dependency (850) except at or near the calibration point (e.g., 25° C.). However, according one aspect of the present disclosure, either one of both of V_(BE1) and V_(BE2) can be adjusted (such as amplified) relative to one another to achieve a more linear dV_(BE). For example, in one embodiment, V_(BE1) is adjusted (e.g., by amplification) relative to V-_(BE2) such that the V_(BE)-temperature curve (810) is effectively moved (rotated) to an adjusted curve, V_(BE1)′-temperature (830). The differential voltage, dV_(BE)′=V_(BE2)−V_(BE1)′, has a more linear temperature dependency, as shown by the dV_(BE)′-temperature curve (860) in FIG. 8B.

As a specific example, in the BJT pair (120) in FIG. 1A, as discussed above, the differential voltage is given by

${{dV}_{BE} = {{V_{{BE}\; 1} - V_{B\; E\; 2}} = {\frac{\eta\;{kT}}{q}\ln\; r}}},$ where r is the current density ratio between the two branches. In the case in which the currents through both diode-connected transistors are the same, the current density ratio is inverse of the emitter area ratio, n/m=N. If η is not a constant with temperature, dV_(BE) is not PTAT. However, according to one aspect of the present disclosure, V_(BE1) and V_(BE2) can be amplified differently to make dV_(BE) be closer to PTAT. For example, V_(BE1) is amplified by a factor C_(s). The differential voltage dV_(BE)=C_(s)V_(BE1)−V_(BE) can be made more PTAT by appropriate choice of C_(s).

Specifically, the current I₁ through the diode-connected transistor (124) in the first branch (120) is

$I_{1} = {I_{s}e^{\frac{{qV}_{{BE}\; 1}}{\eta\;{kT}}}}$ where, I_(s) is a process-dependent saturation current; the current I₂ through the diode-connected transistor (134) in the second branch (130) is

$I_{2} = {{N \cdot I_{s}}{e^{\frac{{qV}_{{BE}\; 2}}{\eta\;{kT}}}.}}$ Amplifying V_(BE1) by C_(s) is equivalent to having a current I₁ ^(Cs):

${I_{1}^{C_{s}} = {I_{s}^{C_{s}}{e^{\frac{{qC}_{s}V_{{BE}\; 1}}{{\eta\;{kT}}\;}}.{Thus}}}},{\frac{I_{1}^{C_{s}}}{I_{2}} = {\frac{I_{s}^{C_{s} - 1}}{N}e^{\frac{q{({{C_{s}V_{{BE}\; 1}} - V_{B\; E\; 2}})}}{\eta\;{kT}}}}},$ and the differential voltage is

${dV}_{BE} = {{{C_{s}V_{{BE}\; 1}} - V_{{BE}\; 2}} = {\ln\;{\frac{{NI}_{1}^{C_{s}}}{I_{2}I_{s}^{C_{s} - 1}} \cdot \frac{\eta\; k}{q} \cdot {T.}}}}$ For the case where I₁=I₂=I,

${{dV}_{BE} = {{{\ln\left\lbrack {N\left( \frac{I}{I_{s}} \right)}^{C_{s} - 1} \right\rbrack} \cdot \frac{\eta\; k}{q} \cdot T} = {C^{\prime\prime\prime}T}}},$ where C′″, the coefficient (or “slope”) in the dV_(BE)-T relationship, is

$C^{\prime\prime\prime} = {{\ln\left\lbrack {N\left( \frac{I}{I_{s}} \right)}^{C_{s} - 1} \right\rbrack} \cdot {\frac{\eta\; k}{q}.}}$

Thus, the coefficient is modified from

$\ln\;{N \cdot \frac{\eta\; k}{q}}$ (i.e., C_(s)=1) for dV_(BE)=V_(BE1)−V_(BE2) to

${\ln\left\lbrack {N\left( \frac{I}{I_{s}} \right)}^{C_{s} - 1} \right\rbrack} \cdot \frac{\eta\; k}{q}$ for dV_(BE)=C_(s)V_(BE1)−V_(BE2). By appropriate choices of C_(s) and I (or I₁ and I₂), the introduction of the factor

$\left( \frac{I}{I_{s}} \right)^{C_{s} - 1}$ can serve to compensate for the temperature dependency of the ideality factor, η, such that the coefficient, C′″, as a whole is less temperature dependent than η, and dV_(BE) is more PTAT, especially in the temperature range of intended operation.

More generally, V_(BE1) and V_(BE2) can be treated differently from each other in other ways than by simple amplification. For example, amplifiers for V_(BE1) and V_(BE2) can be configured such that C_(s) is a temperature-dependent factor instead of a constant.

The linearization of the dV_(BE)-T relationship outlined above can be implemented by a variety of circuits. For example, as shown in FIG. 9, discrete-timing type circuits, such as those shown in FIG. 3, can be used to amplify V_(BE1) and V_(BE2) differently. In this example, the thermal sensor (900) includes two BJT branches (920, 930). The first branch (920) includes a first current source (922) and first diode-connected transistor (924); the second branch (930) includes a second current source (932) and second diode-connected transistor (934). A signal processing circuit in this example includes a pair of switched-capacitor amplifiers (970, 980) of the same types as the switched-capacitor amplifiers (310, 340) shown in FIG. 3. The amplifiers (970, 980) are connected to receive as inputs emitter voltages V_(BE1) and V_(BE2) from the respective branches (920, 930). The amplifiers (970, 980) have different gains, S₁ and S₂, respectively. The outputs of the amplifiers (970, 980) provides the differential signal, V_(BE). S₁ and S₂ can be chosen to produce a dV_(BE) that is satisfactorily PTAT within the intended operational temperature range, e.g., from about −40° C. to about 125° C.

In another embodiment, as shown in FIG. 10, a continues-DC type thermal sensor (1000) includes two BJT branches (1020, 1030). The first branch (1020) includes a first current source, which in this example is a MOSFET M1 (1022), and first diode-connected transistor (1024); the second branch (1030) includes a second current source, which in this example is a MOSFET M2 (1032), and second diode-connected transistor (1034). M1 and M2 form current mirrors with another transistor, which in this example is a MOSFET M3 (1040), and supply the same current as the current, I₁, through M3. The emitter voltage, V_(BE1), of the first transistor (1024) is applied to an amplifier, which in this example is a voltage divider consisting of serially connected resistors R1 (1052) and R2 (1054) and has a gain of C_(s)=R1/(R1+R2). The output voltage, V₁, of the first branch (1020) is taken at the junction between R1 and R2. The output voltage, V₂, of the second branch (1030) is taken at the emitter of the second transistor (1034), i.e., V₂=V_(BE2). The differential output voltage of the thermal sensor (1000) is dV_(BE)=V₁−V₂.

In another embodiment, as shown in FIG. 11, a continuous-DC type thermal sensor (1100) is otherwise identical to the sensor (1000) in FIG. 10, except that a driver (voltage follower) (1160) is used to supply V_(BE1) to the voltage divider formed by the serially connected resistors R1 (1052) and R2 (1054).

In another embodiment, as shown in FIG. 12, a continuous-DC type thermal sensor (1200) is similar to the thermal sensors (1000, 1100) shown in FIGS. 10 and 11, except that the first branch (1220) includes two sub-branches (1220A, 1220B). The sub-branches (1220A, 1220B) each include a first current source, which in this example is a MOSFET M1 or M2 (1222A, 1222B), and first diode-connected transistor (1224A, 1224B); the second branch (1230) includes a second current source, which in this example is a MOSFET M3 (1232), and second diode-connected transistor (1234). M1, M2 and M3 form current mirrors with another transistor, which in this example is a MOSFET M4 (1240), and supply the same current as the current, I₁, through M4. In this example, the current densities in the first and second sub-branches (1220A, 1220B) are the same but are different from the current density in the second branch (1230). The emitter voltage, V_(BE1), of the first transistor (1224A) is applied to a coefficient (C_(s)) generator, i.e., an amplifier (for example, a voltage divider) (1252). The output of the coefficient generator (1252) is fine-tuned by an V_(BE) Adjustment circuit (1256), which can be an amplifier, such as a variable resistor, which in combination with a resistor in the voltage divider (1252) can adjust the output voltage of the voltage divider. The output voltage, V₁, of the V_(BE) Adjustment circuit (1256) is connected to the emitter of the first diode-connected transistor (1224B). The output voltage, V₂, of the second branch (1230) is taken at the emitter of the second transistor (1234), i.e., V₂=V_(BE2). The differential output voltage of the thermal sensor (1200) is dV_(BE)=V₁−V₂.

In another embodiment, as shown in FIG. 13, in a thermal sensor (1300) according to an aspect of the present disclosure, a single diode-connected transistor (1324) is used to alternately generate V_(BE)'s under different bias currents, and the alternately generated V_(BE)'s are combined to generate a signal that is substantially PTAT.

In this embodiment, two current sources (1322, 1332) are connected to the emitter of the diode-connected transistor (1324) via respective switches (e.g., switching transistors) SW1 and SW2 (1326, 1336). The current sources (1322, 1332) supplies different current levels to the diode-connected transistor (1324). A V_(BE) adjustment circuit (1356) is connected across the diode-connected transistor (1324) through a switch (e.g., switching transistor) SW3 (1358). The switches SW1, SW2 and SW3 (1326, 1336, 1358) are operated by a control signal (1360) from a processing unit (not shown), such as a microcontroller unit (“MCU”). The emitter voltage of the diode-connected transistor (1324) is input into an analog-to-digital converter (“ADC”); the output of the ADC is connected to a possessor, which can be the processing unit or a separate processing unit.

In operation according to one embodiment, the processing unit outputs a control signal (1360) to alternately turn SW1 and SW2 on to alternately supply two different currents to the diode-connected transistor (1324). The processing unit outputs a control signal (1360) to turn SW3 on while SW1 is turned on or while SW2 is turned on. Thus, depending on the control signal (1360), four V_(BE)'s can be generated that can be used to produce a substantially PTAT temperature signal: V_(BE1) is the emitter voltage generated by the first current source (1322) without V_(BE) adjustment; V_(BE2) is the emitter voltage generated by the second current source (1332) without V_(BE) adjustment; V_(BE3) is the emitter voltage generated by the first current source (1322) with V_(BE) adjustment; and V_(BE4) is the emitter voltage generated by the second current source (1332) with V_(BE) adjustment. A processor (not shown) receiving from the ADC (1380) the digital representations of V_(BE2) and V_(BE3) can compute dV_(BE)=V_(BE3)−V_(BE2) which, with an appropriate setting of the V_(BE) adjustment circuit, is substantially PTAT. Alternatively, the processor can receive from the ADC (1380) the digital representations of V_(BE1) and V_(BE4) and compute dV_(BE)=V_(BE4)−V_(BE1) which, with an appropriate setting of the V_(BE) adjustment circuit, is substantially PTAT. The switch combinations and corresponding dV_(BE) obtained are summarized in the table below:

SW1 SW2 SW3 V_(BE) dV_(BE) On Off On V_(BE3) V_(BE3) − V_(BE2) Off On Off V_(BE2) On Off Off V_(BE1) V_(BE4) − V_(BE1) Off On On V_(BE4)

FIG. 14 shows the temperature error over a temperature range from −40° C. to 125° C. at the TT process corner of a prototype thermal sensor of FIG. 11, with a single-point calibration at 25° C. As can be seen, the temperature error is within 2° C.

While BJTs are used to generate temperature-dependent voltages in many of the thermal sensors described above, other devices having temperature-dependent outputs can be used. For example, any bandgap thermal sensing device can be used. As an example, in some embodiments, diodes are used instead of BJT transistors. In other examples, field-effect transistors (FET's), such as MOSFET's, can be used instead of BJTs in all thermal sensors illustrated in FIGS. 1A, 3-6 and 9-13. More specifically, FETs operating in subthreshold conditions can be used. In a more specific example, as shown in FIG. 15, a thermal sensor (1500) is essentially the same as the one (500) illustrated in FIG. 5; the difference is that the diode-connected BJTs (524, 534, 554, 564) in the sensor (500) in FIG. 5 are replaced with diode-connected MOSFETs (1524, 1534, 1554, 1564), operating under subthreshold conditions, in the sensor (1500) in FIG. 15.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

The invention claimed is:
 1. A thermal sensor, comprising: a first temperature-sensitive device comprising a first pair of bandgap thermal sensing devices, each adapted to generate a respective temperature-dependent voltage, the first temperature-sensitive device being adapted to generate a first temperature-dependent signal from the temperature-dependent voltages of the first pair of bandgap thermal sensing devices; a second temperature-sensitive device comprising a second pair of bandgap thermal sensing devices and adapted to generate respective temperature-dependent voltage, the second temperature-sensitive device being adapted to generate a second temperature-dependent signal from the temperature-dependent voltages of the second pair of bandgap thermal sensing devices; and a signal processing circuit operatively connected to receive from the first and second temperature-sensitive devices the first and second temperature-dependent signals, and adapted to process the received signals using different processing parameters from each other to generating a first and second processed signals, respectively, and to generate an output signal based on a difference between the first and second processed signals, wherein a linear approximation of the first processed signal as a function of absolute temperature over the temperature range has a first offset at a reference temperature, and a linear approximation of the second processed signal as a function of absolute temperature over the temperature range has a second offset at the reference temperature, and the first and second offsets are substantially the same.
 2. The thermal sensor of claim 1, wherein the signal processing circuit is adapted to process the signal received from the first temperature-sensitive device with a first gain factor and process the signal received from the second temperature-sensitive device with a second gain factor, the second gain factor being different from the first gain factor, wherein the output signal of the signal processing circuit is a differential signal between the processed signals.
 3. The thermal sensor of claim 1, wherein each of the first and second temperature-sensitive devices comprises two branch circuits, each of the two branch circuits comprising a respective one of the bandgap thermal sensing device and a current source adapted to pass a current through the respective bandgap thermal sensing device, each of the bandgap thermal sensing device adapted to generate a signal responsive to the current passed through the bandgap thermal sensing device.
 4. The thermal sensor of claim 3, wherein the first temperature-dependent signal is indicative of a difference between the signals generated by the two bandgap thermal sensing devices in the first branch, and the second temperature-dependent signal is indicative of a difference between the signals generated by the two bandgap thermal sensing devices in the second branch.
 5. The thermal sensor of claim 3, wherein, in each of the first and second temperature-sensitive devices, each of the bandgap thermal sensing devices comprises a bipolar-junction transistor, wherein the current source and bipolar-junction transistor in each branch are configured to generate a current density in the bipolar-junction transistor, and wherein the current densities are different between the two branches.
 6. The thermal sensor of claim 3, wherein, in each of the first and second temperature-sensitive devices, each of the bandgap thermal sensing devices comprises a field-effect transistor, wherein the current source and field-effect transistor in each branch are configured to generate a current density in the field-effect transistor, and wherein the current densities are different between the two branches.
 7. The thermal sensor of claim 1, wherein each of the first and second temperature-sensitive devices comprises a respective one of the pairs of bandgap thermal sensing devices and a current source adapted to pass a current through the respective bandgap thermal sensing device, the bandgap thermal sensing device adapted to generate a signal responsive to the current passed through the bandgap thermal sensing device, wherein the signal processing circuit is adapted to process the signal generated by the bandgap thermal sensing device—with a first gain factor and process the signal generated by the second bandgap thermal sensing device with a second gain factor, the second gain factor being different from the first gain factor, wherein the output signal of the signal processing circuit is a differential signal between the processed signals.
 8. The thermal sensor of claim 1, wherein the output signal is substantially proportional to absolute temperature at the thermal sensor in a temperature range from about −50° C. to about 150° C.
 9. The thermal sensor of claim 8, wherein each temperature corresponding to the output signal within the temperature range differs by no more than 5° C. from a temperature determined by proportional relationship between absolute temperature and output signal.
 10. The thermal sensor of claim 3, wherein one of the two branch circuits in at least one of the first and second temperature-sensitive devices further comprises a voltage divider having an input connected across the bandgap thermal sensing device in the respective branch circuit to receive the signal generated by the respective bandgap thermal sensing device, and an output adapted to supply an output signal that is a fraction of the signal received by the voltage divider, the output signals from the voltage divider being one of the first and second temperature-dependent signals received by the signal processing circuit.
 11. The thermal sensor of claim 1, wherein the signal processing circuit comprises a discrete-timing-type amplifier.
 12. The thermal sensor of claim 1, wherein the signal processing circuit comprises a continuous-DC-type amplifier.
 13. The thermal sensor of claim 1, wherein the signal processing circuit comprises a first switched-capacitor amplifier having an input operatively connected to receive the first temperature-dependent signal, and a second switched-capacitor amplifier having an input operatively connected to receive the second temperature-dependent signal.
 14. The thermal sensor of claim 13, wherein the switched-capacitor amplifiers each having an output, the thermal sensor further comprising a controller adapted to generate from the outputs a signal indicative of the temperature.
 15. A method of temperature measurement, comprising: generating a first current density in a first thermal-sensing device disposed at a temperature, T, to generate a first voltage across the first thermal-sensing device; generating a second current density in a second thermal-sensing device disposed at T, to generate a second voltage across the second thermal-sensing device, the second current density being different from the first current density; generating a third current density in a third thermal-sensing device disposed at T, to generate a third voltage across the third thermal-sensing device; generating a fourth current density in a fourth thermal-sensing device disposed at T, to generate a fourth voltage across the fourth thermal-sensing device, the fourth current density being different from the third current density; obtaining a first differential voltage between a voltage corresponding to the first voltage and a voltage corresponding to the second voltage; obtaining a second differential voltage between a voltage corresponding to the third voltage and a voltage corresponding to the fourth voltage; obtaining a third differential voltage, dV, between the first differential voltage multiplied by a first gain factor and the second differential voltage multiplied by a second gain factor, the second gain factor being different from the first gain factor; and determining T based on the third differential voltage; wherein: a linear approximation of the product between the first differential voltage and the first gain factor as a function of absolute temperature over a temperature range has a first offset at a reference temperature; a linear approximation of the product between the second differential voltage and the second gain factor as a function of absolute temperature over a temperature range has a second offset at the reference temperature; and the obtaining the third differential voltage comprises selecting the first and second gain factors such that the first and second offsets are substantially the same.
 16. The method of claim 15, wherein the determining the temperature, T, comprises: obtaining a value, dV₀, of the third differential voltage at a single, known temperature, T₀; and determining the temperature, T, as T=dV·T₀/dV₀, wherein T and T₀ are measured in K.
 17. The method of claim 16, wherein the voltage corresponding to the first voltage is a non-unity fraction of the first voltage.
 18. A method of temperature measurement, comprising: generating a first current density in a first thermal-sensing device disposed at a temperature, T, to generate a first voltage responsive to the first current density and T; generating a second current density in a second thermal-sensing device disposed at T, to generate a second voltage responsive to the second current density and T, the second current density being different from the first current density; obtaining a differential voltage, dV, between a product between a voltage corresponding to the first voltage and a first gain factor on one hand, and a product between a voltage corresponding to the second voltage and a second gain factor on the other, the second gain factor being different from the first gain factor, wherein the voltage corresponding to the first voltage is a non-unity fraction of the first voltage; and determining T at least in part based on the differential voltage.
 19. The method of claim 18, wherein the determining the temperature, T, comprises: obtaining a value, dV₀, of the differential voltage at a single, known temperature, T₀; and determining the temperature, T, as a function of dV·T₀/dV₀, wherein T and T₀ are measured in K.
 20. The method of claim 19, wherein the determining T as a function of dV·T₀/dV₀ comprises determining a differential voltage between dV·T₀/dV₀ and a signal from at least a third temperature-sensitive device disposed at T. 